Bicycle - Split deck
The magician shows a deck cut diagonally. Spreading the two halves of the deck on the table it shows how the cards are completely different.
The cards are turned over on the back and two spectators are asked to choose each half card from the deck in front of them. The two half-cards are drawn closer and it is shown how the backs match perfectly.
Magically when the magician turns the two halves also the faces coincide, forming a whole card.